1. Field of the Invention
The present invention relates generally to the field of active semiconductor devices. More specifically, the present invention relates to novel semiconductor device structures useful in low voltage and high current density applications. More particularly, the present invention relates to active semiconductor devices referred to as normally off Field Effect Transistors (FET), which specifically include Junction Field Effect Transistors (JFET) as well as Metal Semiconductor Field Effect Transistors (MESFET).
2. Related Art
The increasing trend toward lower supply voltages for active semiconductor devices and Integrated Circuits (IC""s) has accelerated the search for more efficient low voltage power sources. Conventional power supplies utilizing silicon diode rectifiers are unacceptable in low voltage applications due to the excessive voltage drop across the forward biased diode terminals. Power loss in the diodes becomes excessive when they are used as rectifiers in a direct current (dc) power supply designed for a terminal voltage as low as 3.0 volts.
Semiconductor diodes are combined with active devices to form circuits capable of producing low value dc supply voltages, but such circuits are generally not capable of handling the large currents frequently required. They usually exhibit a fairly large internal resistance and as such are very inefficient power sources. Furthermore, the number and complexity of steps required in the processing of this type of circuit as an IC also increases with the number of devices included.
Active semiconductor devices are used as switches in circuit arrangements producing dc power supply voltages, as for example in switched mode power supplies. Junction Field Effect Transistors (JFET) can be used as switches because they are easily switched between an on or conducting state and an off or non-conducting state. Most importantly, the current carriers in a JFET are all majority carriers which results in short switching times. However, when operated at lower voltages, JFETs exhibit an internal resistance in the on state that make them unsatisfactory and inefficient in applications requiring large currents.
In U.S. Pat. No. 4,523,111 entitled xe2x80x9cNormally-Off Gate-Controlled Electric Circuit with Low On-Resistancexe2x80x9d, Baliga disclosed a JFET serially connected to an Insulated Gate Field Effect Transistor (IGFET). The on resistance of this circuit is the sum of the JFET resistance and the IGFET resistance. As a result, the on resistance is too large and therefore unsatisfactory for low voltage operations requiring large currents.
In a similar invention disclosed in U.S. Pat. No. 4,645,957 entitled xe2x80x9cNormally Off Semiconductor Device with Low On-Resistance and Circuit Analoguexe2x80x9d by Baliga, a JFET is serially connected to a Bipolar Junction Transistor (BJT). The on resistance is the sum of the JFET and the BJT which is again too large for low voltage applications requiring large currents.
The previously cited U.S. patent application Ser. No. 09/430,500, xe2x80x9cNOVEL JFET STRUCTURE AND MANUFACTURING METHOD FOR LOW ON RESISTANCE AND LOW VOLTAGE APPLICATIONSxe2x80x9d, Ho-Yuan Yu, filed Dec. 2, 1999, discloses the basic structure for novel semiconductor devices useful for switching high level currents in ac circuit applications. These novel semiconductor devices have very low on resistance, and could be useful as switches in circuit arrangements producing dc power supply voltages, as for example in switched mode power supplies. Furthermore, the current carriers in these devices are all majority carriers which would result in short switching times. However, in dc circuit applications at voltage levels greater than approximately 0.4 volts, the normally off JFET disclosed will not easily switch between an on or conducting state and an off or non-conducting state. The normally off JFET is not easily used as an amplifier under dc bias above 0.4 volts. Therefore a need of starter device to assist normally off JFET to be used as a switch or an amplifier under dc bias above 0.4 volt.
Accordingly, what is needed is a semiconductor circuit that can efficiently supply the dc currents required in both discrete and integrated circuits being operated at low dc supply voltages. What is also needed is a semiconductor switching device or an amplifier that has a very low on or current conducting resistance. What is needed yet is a semiconductor switching device or an amplifier that can be easily switched between an on or current conducting state and an off or non-current conducting state with the smallest possible switching time. What is further needed is a circuit or method that will allow the use of a normally off FET in dc circuit applications at dc voltage levels greater than approximately 0.4 volts. The present invention provides these advantages and others not specifically mentioned above but described in the sections to follow.
A semiconductor switching device or an amplifier combined in parallel with one or more active devices defined as a starter device. A starter device is used to reduce the terminal voltage of a switching device to a dc level below about 0.4 volts which will then allow the switching device to transition between the on or conducting state and the off or non-conducting state. The starter device also allows normally off JFET to be used as an amplifier under dc bias greater than 0.4 volt. Three different starter devices are utilized. The first being a Bipolar Junction Transistor (BJT), the second a Metal Oxide Silicon Field Effect Transistor (MOSFET), and the third consisting of three normally off JFETs connected serially. In general, a single starter device is coupled with the terminals of a semiconductor switching device, but it is possible and sometimes advantageous to couple two or more starter devices in parallel. In a first case, a symmetrical, normally off or enhancement mode JFET is used as the semiconductor switching device. One or more starter devices coupled between source and drain of the JFET will allow switching at dc voltage levels greater than 0.4 volts. In a second case, an asymmetrical, normally off JFET is used as the switching device. One or more starter devices coupled between source and drain of the JFET will allow switching at dc voltage levels greater than 0.4 volts. In a third case, a normally off MESFET is used as the switching device. One or more starter devices coupled between source and drain of the MESFET will allow switching at dc voltage levels greater than 0.4 volts.
More specifically, an embodiment of the present invention includes a symmetrical, enhancement mode JFET as the switching device or an amplifier device. In a first case, a BJT acting as the starter device is coupled between source and drain of the JFET. This BJT can be designed along with enhancement mode JFET or use the prasitic BJT of JFET as the starter device. In a second case, a normally off MOSFET acting as the starter device is coupled between source and drain of the JFET. In a third case, three normally off JFETs connected serially as a starter device are then coupled between source and drain of the JFET. Further cases include two or more starter devices coupled between source and drain of the JFET. Each of the resulting structures provide high current carrying capacity at low voltage levels, and will easily switch between states at dc voltage levels greater than 0.4 volts or used as an amplifier at dc voltage levels greater than 0.4 volt.
A second embodiment of the present invention includes an asymmetrical, enhancement mode JFET as the switching device or an amplifier device. In a first case, a BJT acting as the starter device, either by added structure or use its parasitic BJT structure, is coupled between source and drain of the JFET. In a second case, a normally off MOSFET acting as the starter device is coupled between source and drain of the JFET. In a third case, three normally off JFETs connected serially as a starter device are then coupled between source and drain of the JFET. Further cases include two or more starter devices coupled between source and drain of the JFET. Each of the resulting structures provide high current carrying capacity at low voltage levels, and will easily switch between states at dc voltage levels greater than 0.4 volts.
A third embodiment of the present invention includes a symmetrical, enhancement mode MESFET as the switching device or an amplifier device. In a first case, a BJT acting as the starter device by added structure is coupled between source and drain of the MESFET. In a second case, a normally off MOSFET acting as the starter device is coupled between source and drain of the MESFET. In a third case, three normally off JFETs connected serially as a starter device are then coupled between source and drain of the MESFET. Further cases include two or more starter devices coupled between source and drain of the MESFET. Each of the resulting structures provide high current carrying capacity at low voltage levels, and will easily switch between states at dc voltage levels greater than 0.4 volts.